The present invention relates to x-ray generating apparatus, and in particular to x-ray tubes with an improved unitary vacuum housing design which allows for a radiation protection and direct heat transmission through a body of the unitary vacuum housing.
The x-ray generating apparatus generally comprises a vacuum enclosure with an anode assembly and a cathode assembly spaced therebetween. The cathode assembly comprises an electron emitting cathode which is disposed so as to direct a beam of electrons onto a focal spot of an anode target of the anode assembly. In operation, electrons emitting by the cathode are accelerated towards the anode target by a high voltage created between the cathode and the anode target. The accelerated electrons impinge on the focal spot area of the anode target with sufficient kinetic energy to generate a beam of x-rays which passes through a window in the vacuum enclosure.
However, only about one percent of the input energy is converted into x-radiation. The vast majority of the input energy is converted into thermal energy which is stored in the mass of the anode assembly. It is known in the art that by rotating the anode the heat generated during x-ray production can be spread over a larger anode target area. To improve the heat transfer by radiation the anode assembly is coated in a special way and is cooled by forced convection with, for example, a dielectric liquid as disclosed in the U.S. Pat. No. 4,928,296. The excessive thermal energy from the anode assembly is dissipated by thermal radiation to the surrounding enclosure.
In conventionally designed x-ray generating apparatus the vacuum enclosure is placed in a housing which serves as a container for cooling medium, typically cooling fluid or the forced air. In fluid cooled x-ray apparatus, the type disclosed for example in the U.S. Pat. No. 4,841,557, the rotating anode x-ray tube is immersed into the housing filled with an insulating fluid such as a transformer oil which is circulated by a pump for at least partially dissipating the heat from the vacuum enclosure.
The air cooled x-ray tube disclosed in the U.S. Pat. No. 5,056,126 comprises a housing with disposed therein an evacuated envelope having a cathode and an anode that are capable of being biased to a voltage in a range between about 1 kV and 200 kV, and a heat cage formed of a heat conducting material. The head cage is provided within the interior of the vacuum enclosure surrounding an anode target. The heat cage absorbs heat from the anode and transports it to the end portion of the vacuum enclosure, and then to the exterior of the housing for dissipation by the air flow. The excessive radiation from the x-ray tube is blocked from exiting the housing by a lead liner which is provided between the evacuated envelope and the housing. The lead liner serves also as a massive heat sink for the x-ray tube.
Being advantageous in some respects the air cooled tube design has certain drawbacks. The presence of the heat cage inside the evacuated envelope elongates the heat path leading to a heat dissipation which results in excessive temperature built up over the exterior of the vacuum enclosure which may damage the lead liner.
Therefore it is an object of the present invention to provide a compact x-ray generating apparatus with reduced number of components resulting increased reliability and reduced manufacturing costs.
It is another object of the present invention to provide the x-ray generating apparatus having a multi-functional vacuum enclosure which serves as a radiation shield, as a heat reservoir for balancing the temperature within the vacuum enclosure in case of power loss and as a direct heat transfer element between an anode assembly and an air cooling system.
It is yet another object of the present invention to provide the air cooling x-ray generating apparatus comprising a multi-functional mounting block which serves as an installation element, as a heat reservoir and as an element of a cooling system.
In accordance with one embodiment of the present invention, there is provided an x-ray generating apparatus which comprises a unitary vacuum enclose formed by a cylindrically shaped body having side, top and bottom walls with respective openings therein. The top and side walls are made of materials capable to provide a required radiation shielding which does not exceed the FDA requirement of radiation transmission equal to 100 mRad/hr at 1 meter from the x-ray generating apparatus with 150 kV at rated power. The unitary vacuum enclosure has an anode assembly with a rotating anode target and a cathode assembly spaced therebetween. The unitary enclosure has a thermal capacity that is substantially larger than a thermal capacity of the anode target. The cathode assembly has an electron source for emitting electrons that strikes the rotating anode target to generate x-rays which are released through an x-ray window coupled to the opening in the side wall of the unitary vacuum enclosure. The cathode assembly comprises further a mounting structure for holding said electron source, and a disk made of a high Z-material and attached to the mounting structure and facing the anode target for shielding the opening in the top wall of the unitary vacuum enclosure against the x-rays. The outer side wall of the unitary vacuum enclosure comprises a plurality of fins disposed thereon. A shroud is attached to the fins and extends over the outer perimeter of the side wall and partially over the top wall.
In accordance with another embodiment of the present invention the x-ray generating apparatus comprises a top wall and a cylindrical side wall with a protruded inwardly shielding member. The shielding member is substantially parallel to the top wall. It forms an upper and lower portion within the vacuum enclosure, wherein an anode assembly and an electron source of cathode assembly are disposed in the lower portion, while the mounting structure for holding the electron source is disposed in the upper portion of vacuum enclosure. The cathode assembly is placed within the vacuum enclosure through an aperture within the upper portion of the side wall of the vacuum enclosure. A conical high-voltage insulator is utilized to seal the vacuum enclosure within this aperture.
These and other objectives and advantages of the present invention will become clear from the detailed description given below in which preferred embodiments are described in relation to the drawings. The detailed descriptions presented are to illustrate the present invention, but is not intended to limit it.